1. Field of the Invention
This invention relates to a welding machine and more particularly to method and apparatus for automatically welding together edges of sheet metal to form a sealed joint in the fabrication of duct work.
2. Description of the Prior Art
It is well known in heating and air conditioning installations to convey heated and cooled air to selected areas through duct work of polygonal and circular cross section. The duct work is made of sheet material, preferably sheet metal, of a selected dimension. Polygonal duct work is connectionally square or rectangular in cross section and includes a pair of lateral duct side walls connected to upper and lower duct side walls in a sealed assembly. Selected lengths of duct work are connected in end to end relation by a duct joint assembly, for example as disclosed in U.S. Pat. No. 4,218,079. The duct joint assembly sealingly connects adjacent ends of the duct work to prevent heated or cold air from escaping out of the duct work at the joints.
In the assembly of duct work from sheet metal, the sheet metal members are connected to one another along the adjacent longitudinal edges to form a polygonal or circular cross section. It is well known to connect the adjacent longitudinal edges of the sheet metal members by welding or a mechanical interlocking connection. The mechanical interlocking connection is fabricated by roll forming the longitudinal edges of the sheet metal members in a locking device which permits the longitudinal edge of one sheet metal member to mechanically engage the longitudinal edge of the adjacent sheet metal member. Once the edges are engaged, the engaged edges are locked in place by deforming the engaged edges in locking engagement. In this manner a mechnical seam is formed which is sealed by tape or by the application of gasket material or a resilient sealant. All four edges in a polygonal duct or a pair of edges of duct work bent in a polygonal cross section are interlocked in this manner to form assembled duct work of a preselected length and cross sectional dimension. Selected lengths of duct work are then sealingly connected in end to end relation by a suitable duct joint assembly, such as the duct joint assembly disclosed in U.S. Pat. No. 4,218,079.
One disadvantage of mechanically interlocking the longitudinal edges of sheet metal members is that additional material is required to form the mechanical connection. This adds to the cost and the assembly time of the duct work. Also, roll forming the longitudinal edges of sheet metal requires careful handling of the sheet metal to prevent damage to the roll formed edges. Once the duct work is assembled by this method, additional steps must be taken to provide an airtight joint, further adding to the cost and time in assembly of the duct work.
Welding the longitudinal edges of the sheet metal members together overcomes the above discussed problems encountered with mechanically interlocking the sheet metal members. The sheet metal members are welded along the adjacent longitudinal edges by arc welding a seam extending the longitudinal length of the sheet metal. The duct work is thus assembled with the weld seam on the external portion of the duct work. This method eliminates the requirement of additional material required with the mechanical interlock arrangement and does not require the addition of any sealant material at the seam.
In one method of welding, the adjacent longitudinal edges are connected along their entire length by the weld positioned on the inside of the formed duct work. Consequently, the joint from the external portion of the duct work is clean, i.e. free of a weld seam. However with this method, once the connected L-shaped sheet metal members are formed, the welding electrode must be advanced downwardly inside the positioned sheet metal members to complete the cross sectional connection. In the assembly of duct work having smaller cross sections, forming an internal weld seam is difficult because of the limited work area for passing the welding gun or electrode downwardly within the duct work.
To avoid the constraints of forming polygonal duct work by welding internally within the duct work, it is also known to fabricate duct work by forming a welded joint which is exposed, i.e. on the external edges of the abutting sheet metal members. This is accomplished by a method known as track welding in which the sheet metal members are horizontally clamped into position on a triangularly shaped post to hold the adjacent longitudinal edges in abutting relation. A welding electrode of an arc welding machine is supported on an arm positioned for movement on a track above the abutting sheet metal members. The welding gun is advanced with the arm by a drive mechanism the length of the abutting sheet metal members to form a weld at the abutting longitudinal edge portions.
Duct work formed by the above described track welding operation forms the weld seam on the external portion of the duct work. This is objectionable when clean joints on the external portion of the duct work are desired. Another problem encountered with the track welding operation is maintaining the welding wire in the desired position overlying the abutting edges of the sheet metal members as the welding wire is fed from a reel. If the welding wire is not maintained in precise overlying relation with the abutting edges, then the welded seam will not be complete. Gaps will be formed in the seam resulting in air leakage during the operation of the duct work.
While it has been suggested to assemble duct work by welding sheet metal members together to form either internal or external weld seams, the known methods do not provide automatic welding of sheet metal edges in the fabrication of duct work having clean external joints free of gaps. Therefore, there is need for method and apparatus for automatically welding sheet metal edges together to fabricate duct work having clean external joints in which the welded seam is positioned internally within the assembled duct work.